Sediment Microplastic Pollution in Contrasting Estuarine Environments of the Biobío Region South-Central Chile

Abstract

Human activities have triggered microplastic pollution, and estuaries have emerged as critical yet understudied ecosystems in Chile. This study investigated sediment microplastic pollution in the Lenga (highly industrialized) and Tubul-Raqui (fisheries village) ecosystems, characterized by contrasting anthropogenic impacts, in the Biobío Region, Chile. Microplastic particles, including fibers, foam, fragments, and film, were detected in both estuaries. The Lenga estuary, heavily industrialized, exhibited a significantly higher total abundance of microplastics compared to the Tubul-Raqui estuary. However, the mean concentrations of microplastics in the studied estuaries are notably lower than those reported in other global studies, aligning more closely with levels found in less polluted estuaries around the world. FTIR analyses identified six types of polymers in Lenga, with polyamide (PA) being the most prevalent, constituting 35% of all polymers detected. Conversely, in Tubul-Raqui, polyvinyl chloride (PVC) emerged as the predominant polymer, comprising 25% of the total. Furthermore, significant correlations were observed with sediment physico-chemical parameters, such as organic matter and pH. These findings confirm the existence of microplastic pollution in both estuaries, highlighting the necessity of continued monitoring and assessment of potential environmental impacts in these ecologically valuable ecosystems.

1. Introduction

Due to the remarkable versatility of plastics and their ubiquity across packaging, construction, transport, electronics, healthcare, and consumer goods, global production has continued to climb to more than 400 million tons in 2024, despite circularity efforts [1]. However, its high persistence in the environment and inadequate final disposal have resulted in notable pollution, with millions of tons of plastic estimated to enter oceans each year [2]. Therefore, plastic has become a persistent pollutant across numerous environments [3]. Various physical and chemical degradation factors contribute to the fragmentation of plastic into smaller particles. According to the dimensional classification proposed by GESAMP [4], these particles are grouped into megaplastics (>1 m), macroplastics (25–1000 mm), mesoplastics (5–25 mm), microplastics (<5 mm), and nanoplastics (<1 μm), with microplastics (MPs) defined within the range from 1 µm to 5 mm. This phenomenon has emerged as a global environmental concern, impacting both terrestrial and aquatic ecosystems [5,6], with observed direct and indirect effects on flora and fauna [7]. In recent years, studies conducted in Chile have increasingly focused on the topic of microplastics, revealing their presence in various environmental compartments. For example, MPs have been detected in surface waters [8,9], sediments [10], and biota, including commercially important fish (in the digestive tissues) [11], king crabs (Lithodes santolla, Molina 1782) [2], and sea lions (Arctocephalus australis, Zimmermann, 1783) [12]. In addition, Nacaratte et al. [13] recently reported the presence of MPs particles in several brands of bottled water distributed in the country’s capital. There is still a lack of studies addressing the occurrence of MPs and their interaction with sediment [14,15].

Estuaries still lack comprehensive studies on plastic and microplastic pollution. Estuarine sediments serve both as sinks and sources of microplastics influenced by complex hydrodynamic and biogeochemical processes. Microplastics introduced through urban runoff, industrial discharges, and maritime activities are prone to accumulate in low-energy sedimentary areas characterized by enhanced depositional conditions. Recent modeling efforts have demonstrated that sediment-water interactions significantly affect microplastic retention and transport, particularly under varying hydrodynamic conditions such as storms and tidal fluctuations [14]. According to Firdaus et al. [16], the geographic locations of estuaries, identified as an interface zone between the ocean and rivers, make them pollution hot spots. Furthermore, estuaries may serve as a significant filter for plastic pollution, acting as a trap that may have negative consequences for these productive ecosystems. This is particularly concerning given that estuaries are ecologically valuable ecosystems, providing habitats for diverse species such as shellfish, fish, and birds [17,18,19] and a source of economic income for coastal communities. Therefore, this study aims to analyze the MP content in the sediments of two estuaries within the Biobío Region in south-central Chile, each characterized by different levels of anthropogenic impact, to verify the relationship between observed pollution and human activities in their vicinity. Additionally, this research seeks to contribute to advancing our understanding of the local and global challenges associated with MPs pollution.

2. Materials and Methods

2.1. Study Area

The study area consisted of two bodies of water in the coastal area of the Biobío Region, Chile: the Lenga and Tubul-Raqui estuaries (Figure 1). The Lenga estuary (36°46′ S; 73°10′ W) is located on San Vicente Bay and covers an area of 3.2 km², which has been affected by multiple industries, with an oil refinery and steel and chemical plants [20]; the adjacent coastal area has undergone major industrialization, which, along with the economic development of the Biobío watershed, has had a substantial impact on the area, contributing a considerable amount of sediment to the coastal ecosystem [21]. Other human activities, such as the extraction of living resources and urbanization, take place in the proximity of the estuary, where restaurants and tourism development have also been promoted. Meanwhile, the Tubul-Raqui estuary (37°14′ S; 73°26′ W) is located on the southern part of the Gulf of Arauco and is formed by two main tributaries, the Tubul and Raqui systems, which have drainage areas of 98 and 176 km², respectively [22]. These systems are situated in a small coastal watershed and, unlike the Lenga estuary, are not significantly affected by industrial processes, thereby presenting a minimal human influence [23]. Even so, in the vicinity of the estuary, there is a degree of urbanization in the form of a small town, the inhabitants of which are dedicated to fishing activity in the area [24].

2.2. Sample Collection

Sediment samples were collected during the spring of 2022. For each estuary, 18 sampling points were established (Figure 1), at which sediment samples (between 200 and 300 mL of sediment per point) were subsequently collected using a Van Veen grab from the upper 10–20 cm of surface sediment. Once extracted, samples were stored in labeled aluminum foil bags, transported to the laboratory, and refrigerated at 4 °C until analysis.

2.3. Microplastic Extraction

The density separation method was used to obtain the MPs particles from the sediment, following the methodology used by Thompson et al. [25]. As a first step, a supersaturated sodium chloride (NaCl) solution (1.2 kg NaCl L⁻¹) was prepared and then filtered with a filter paper with a 1-µm pore size (Ahlstrom Munksjö, Helsinki, Finland, Grade 393 size 11 cm) to remove any particles present in the solution. Subsequently, 50 mL of sediment was extracted, to which 100 mL of the respective solution was added; this was shaken for 30 s and left to rest for 2 min before proceeding with the supernatant filtration (which was repeated 3 times per sample) using a vacuum filter device and 1.6- µm glass fiber filter paper (glass fiber prefilters, Merck Millipore Ltd., Darmstadt, Germany). Finally, the filters were stored in previously labeled glass Petri dishes and dried at room temperature in a glass box to avoid external contamination.

Once the filters were completely dry, they were analyzed under a stereomicroscope (Olympus SZ61 45X, Tokyo, Japan), and using fine-tipped metal tweezers, the particles with a length under 5 mm were identified, characterized, counted, and extracted; their forms were determined, and they were subsequently stored on slides. Finally, the particles were analyzed using Fourier transform infrared spectroscopy (FTIR) (FT/IR-4600 Jasco, Tokyo, Japan). Transmittance mode was used with a spectral range of 400–4000 cm⁻¹. Thirty-two scans of each analyzed sample were performed, with a resolution of 8 cm⁻¹. The obtained spectra were processed with the search software KnowItAll System 2020, JASCO Spectroscopy Edition, and compared to databases of spectral libraries such as Wiley, Sadtler, and Raman, among others, to identify the composition of the polymers. Only particles with a percentage of overlap with the database above 60% were considered MPs [26].

2.4. Physicochemical Parameter Analysis

2.4.1. Particle Size

Subsamples were extracted from the original samples, which were sieved to 2000 μm. Subsequently, a sediment cleaning procedure was carried out to eliminate organic matter, carbonate, and biogenic silica content. Then they were placed in a Mastersizer 3000 to analyze the particle sizes using laser diffraction. Once the results were obtained, the Gradistat v8.0 spreadsheet was used to obtain the sediment particle size distribution.

2.4.2. Organic Matter

To determine the organic matter content, the weight loss on ignition method was used following Heiri et al. [27]. First, the samples were placed in Petri dishes and dried in a drying oven (model ULM 500, Memmert, Schwabach, Germany) at 60 °C until completely dry. Then they were homogenized using a mortar and a gram of sediment was extracted, which was placed in a muffle furnace (model N20/HR, Nabertherm, Lilienthal, Germany) at 550 °C for 4 h. Finally, the samples were weighed again to determine the percentage of organic matter present.

2.4.3. pH

pH was determined based on the method outlined by Mylavarapu et al. [28]. The first step was the extraction of subsamples (25 g) of the original samples, which were stored in previously washed beakers. Then, in brief, 50 mL of Milli-Q water was added to the subsamples to obtain a 2:1 ratio. The mixtures were then shaken using an orbital shaker at 250 rpm for a duration of 30 min. Finally, the pH of the solutions was measured using a pH meter (model HI2020, Hanna Edge, Woonsocket, RI, USA).

2.5. Statistical Analysis

R version 4.2.1 was used for all statistical analyses. First, the Shapiro-Wilk test was conducted to ascertain whether the data from both estuaries—including MPs and physicochemical parameters—followed a normal distribution. Next, the Mann-Whitney U test was applied to corroborate whether there were significant differences between the results of each estuary. In addition, correlations (Spearman correlations) were performed to observe if there were relationships between the forms of MPs (i.e., fibers, fragments, foam, and film) and sediment parameters (particle size, organic matter, and pH) using a significance level of p < 0.05. A principal component analysis (PCA) was also performed to explore data variance in the two estuaries.

3. Results

The particle size distributions in the sediments of the two estuaries were relatively similar, as shown in

Table 1. Sediment physicochemical parameters (mean ± SD) by estuary.

Estuary	n	Particle Size (μm)	Classification	Organic Matter (%)	pH	Moisture (%)
Lenga	18	110.60 ± 48.11	Very fine sand	5.00 ± 4.00	7.89 ± 0.59	66.77 ± 41.63
Tubul-Raqui	18	127.54 ± 53.16	Fine sand	3.43 ± 1.46	7.48 ± 0.30	38.59 ± 11.31

. In the Lenga estuary, particle sizes ranged from 28.84 to 168.6 μm among the individual samples, with most points presenting fine sand and very coarse silt predominating at only a few locations. Meanwhile, in the Tubul-Raqui estuary (59.84–223.2 μm), fine and very fine sand predominated. Each sample corresponded to a single measurement, with no internal variability, and the detailed individual values are presented in Appendix A.

3.1. Microplastic Abundance and Concentration

In the Lenga estuary, a total of 119 particles were found, corresponding to the sum of the 18 individual sediment samples collected, with FTIR analysis confirming that only 94 (79%) were MPs, while the remaining 25 (21%) were other types of particles. MPs particles were found to be distributed among almost all sampling points (n = 17/18), at which ranges of 2–10 particles/50 mL sediment per point were observed, with an average abundance of 107 ± 67 particles/kg d. w. Four forms of MPs were found (Figure 2), of which fibers presented the greatest frequency, accounting for 46.8% of the total, followed by foam (39.4%), fragments (7.4%), and film (6.4%).

Meanwhile, in the Tubul-Raqui estuary, the total count of particles occurring at the sampling points revealed a total abundance of 63 particles, of which only 56 (89%) were MPs, while the remaining 7 (11%) were identified as other types of particles. The MPs particles were found to be present at almost all sampling points (n = 15/18), where there was a variation of 2–7 particles/50 mL sediment, with an average abundance of 49 ± 31 particles/kg d. w. As in the Lenga estuary and as seen in Figure 2, four forms of MPs were observed, with fibers and foam found at the same frequency (46.4%), while fragments (5.4%) and film (1.8%) were found in lower quantities.

A comparison of the results from each estuary reveals significant differences in total MPs abundance (W = 243, p = 0.0100), as well as fiber concentrations (W = 223.5, p = 0.0469), which in both cases were notably greater in Lenga. Regarding the other forms of MPs (fragments, foam, film), no significant differences were found.

3.2. Microplastic Polymer Types

FTIR analysis provided the respective spectra of the MPs, which were used to ascertain their composition; some examples can be seen in Figure 3.

In the Lenga estuary, six distinct polymer types were identified (Figure 4), of which polyamide (PA) was the most abundant (35%), followed by polyester (PE) (26%) and polyurethane (PU) (19%). Six types of non-MP particles were also identified, of which styrene-grafted cotton presented the greatest abundance (n = 7), followed by polyamic acid particles and cellulose fibers, each with a total of six particles, phenolic compound (n = 3), epoxy compound (n = 2), and styrene-divinylbenzene (n = 1).

Meanwhile, in the Tubul-Raqui estuary eight polymer types were identified (Figure 5), with polyvinyl chloride (PVC) predominating (25%), followed by polyamide (PA) (16%) and polymer compound (CP), which also accounted for 16% of the total and was described in the database as a weatherproof protective polymer compound used to coat and seal insulation; therefore, it was classified as a polymer as such. Meanwhile, four non-MP particle types were identified, of which the most abundant was styrene-grafted cotton (n = 4), followed by polyamic acid (n = 1), epoxy compound (n = 1), and styrene-divinylbenzene (n = 1).

3.3. Principal Component Analysis (PCA)

The principal component analysis (PCA) was performed using the first two principal components (Figure 6), which accounted for approximately 56% of the data variance. Film, along with the physicochemical parameters, exhibited great variability along the first principal component (PC1) axis, explaining 37.5% of the total data variance. Sediment from the Lenga estuary was associated with a higher presence of film. Meanwhile, the PC2 axis captured approximately 18.9% of the variance, showing that fibers and foam presented the greatest variability.

It was observed that some forms of MPs were correlated with sediment physicochemical variables. Organic matter content was positively correlated with fragments (r_s = 0.447, p = 0.0063) and film (r_s = 0.355, p = 0.0334). Likewise, film concentrations were positively correlated with pH (r_s = 0.443, p = 0.0068) and negatively with particle size (r_s = −0.362, p = 0.03). Regarding fiber and foam concentrations, no significant correlations with the physicochemical variables were found.

4. Discussion

Fibers and foam predominated in both estuaries. The greater fiber abundance is a pattern identified not only in this study but also in others worldwide. For example, a study conducted by Sánchez-Hernández et al. [29] in the Tecolutla estuary in México found that fibers accounted for approximately 72% of the MPs found in the sediment, with the authors noting that the high abundances in the estuary were due to the large quantities of waste received from activities such as fishing, industry, and tourism, among others. A similar situation was found in China in the sediment of two estuaries—Changjiang [30], Yangtze [26], where fibers were the most abundant form of MPs. According to Vásquez-Molano et al. [31], high MP fiber content in sediment could be related to, for example, fishing nets and correlated with the discharge of partially treated wastewater, both of which are consistent with the anthropogenic activities observed in the estuaries examined in this study. Several studies have identified washing synthetic textiles as a major pathway for fiber release into aquatic environments, due to the fragmentation during laundering and the discharge of domestic effluents [6,32,33,34,35]. Therefore, the washing of synthetic textiles could also have a significant impact on the study area.

For the mean concentrations determined in the estuaries, despite differences between them, their levels are notably lower compared to other studies conducted worldwide (Table 2). For example, in the sediments of the Wanquan River estuary in China, 1065 ± 696 particles/kg d.w. were reported [5], as well as in the estuaries of India (1580 ± 563 particles/kg d.w.) [36], Vietnam (2188 ± 1499 particles/kg d.w.) [37], Argentina (1693 ± 2315 particles/kg d.w.) [38] and France, where the mean was 4440 particles/kg d.w. in summer and 1443 particles/kg d.w. in winter [39], with both Wanquan River and other estuaries showing mean concentrations exceeding 1000 particles/kg d.w. The differences may be linked to various factors that promote plastic waste pollution and subsequently the generation and accumulation of MPs, such as human settlements, population density, wastewater discharges, climatic factors, and the hydrodynamics of each estuary, among others. Despite the significant difference in MPs particle levels between the previous studies and the estuaries in this study, other estuaries have shown similar contamination levels. For instance, Xu et al. [40] reported a mean concentration of 120 ± 46 particles/kg d.w. in the Liaohe estuary, China, and Sánchez-Hernández et al. [29] documented 121 particles/kg d.w. in the Tecolutla estuary, Mexico, concentrations that are relatively similar to those reported in the Lenga estuary (106.9 ± 67.2 particles/kg d.w). However, the mean concentration documented in the Tubul-Raqui estuary (49.3 ± 30.9 particles/kg d.w) remains lower compared to other estuaries. This aligns with other studies, such as that by Nithin et al. [41], who analyzed the Uppanar and Gadilam estuaries in India, reporting 36.3 ± 3.39 particles/kg d.w. and 51.6 ± 2.05 particles/kg d.w., respectively.

Within the Latin American context, in addition to the studies conducted in Argentina and Mexico, other investigations have also documented MPs in estuarine sediments from various countries. For example, in Colombia, the mangroves of two remote tropical estuaries, Saija and Timbiqui, recorded 190 and 208 plastic particles, respectively, during the high rainfall seasons, with MPs being the predominant size category (50–87%) compared to meso- and macroplastics [42]. Similarly, in the Chubut River estuary in Argentina, an average of 175.4 ± 63.5 anthropogenic particles/kg d.w. was reported, including MPs and other types of fibers [43]. Another study carried out in Ecuador, in the Salado estuary, reported concentrations ranging from 720 to 7090 particles/kg [44]. In Brazil, in Guanabara Bay, a mean concentration of 528.0 ± 30.0 particles/kg was reported [45]. These findings indicate that, even within the Latin American context—where geographical and socio-environmental variables could be considered more comparable than with other regions of the world—the reported concentrations still exhibit substantial variability, placing the estuaries assessed in this study within the lower range of MPs contamination.

Table 2. Reported concentrations of microplastics in sediments from different estuaries on a global scale.

Estuary	Microplastics Concentration (Particles/kg Sediment d.w.)	References
Wanquan River, China	1065.0 ± 696.0	[5]
Jagir, Indonesia	92.0–590.0	[16]
Kollidam River, India	1580.0 ± 563.0	[36]
Red River, Vietnam	2188.0 ± 1499.0	[37]
Loire, France	4440.0 (summer)–1443.0 (winter)	[39]
Liaohe, China	120.0 ± 46.0	[40]
Uppanar and Gadilam, India	36.3 ± 3.39–51.6 ± 2.05	[41]
Kayamkulam, India	433.3	[46]
Sebou, Morocco	10.0–300.0	[47]
Tecolutla, Mexico	121.0	[29]
Bahia Blanca, Argentina	1693.0 ± 2315.0	[38]
Chubut River, Argentina	175.4 ± 63.5	[43]
Salado, Ecuador	720–7090	[44]
Guanabara Bay, Brazil	528.0 ± 30.0	[45]
Lenga, Chile	106.9 ± 67.2	Present study
Tubul-Raqui, Chile	49.3 ± 30.9	Present study

Table 2. Reported concentrations of microplastics in sediments from different estuaries on a global scale.

Estuary	Microplastics Concentration (Particles/kg Sediment d.w.)	References
Wanquan River, China	1065.0 ± 696.0	[5]
Jagir, Indonesia	92.0–590.0	[16]
Kollidam River, India	1580.0 ± 563.0	[36]
Red River, Vietnam	2188.0 ± 1499.0	[37]
Loire, France	4440.0 (summer)–1443.0 (winter)	[39]
Liaohe, China	120.0 ± 46.0	[40]
Uppanar and Gadilam, India	36.3 ± 3.39–51.6 ± 2.05	[41]
Kayamkulam, India	433.3	[46]
Sebou, Morocco	10.0–300.0	[47]
Tecolutla, Mexico	121.0	[29]
Bahia Blanca, Argentina	1693.0 ± 2315.0	[38]
Chubut River, Argentina	175.4 ± 63.5	[43]
Salado, Ecuador	720–7090	[44]
Guanabara Bay, Brazil	528.0 ± 30.0	[45]
Lenga, Chile	106.9 ± 67.2	Present study
Tubul-Raqui, Chile	49.3 ± 30.9	Present study

Likewise, the notably higher abundance in Lenga may be related to the multiple human activities that take place in its vicinity, especially industry and tourism, with the estuary identified as one that has faced greater anthropogenic pressures than Tubul-Raqui [23]. This difference between the two estuaries could explain why in Lenga, MPs particles were present at more sampling points (94.4%) than in Tubul-Raqui (83.3%).

Even so, MPs pollution was detected in both estuaries, consistent with the findings of Firdaus et al. [16], who noted that estuaries -serving as transport pathways between rivers and oceans—are particularly vulnerable to MP contamination due to their hydrodynamic characteristics. However, this study marks the first time that pollution by these particles has been analyzed in both the Lenga and Tubul-Raqui estuaries, highlighting a lack of prior data for comparative purposes in these areas. Despite the relatively low concentration of microplastics in these estuaries compared to global levels (see Table 2), proactive measures are essential to prevent further contamination, especially given the increasing anthropogenic pressures in the region. Immediate action aimed at reducing plastic waste and improving waste management practices is crucial to safeguarding these ecologically valuable ecosystems.

In the MPs composition, PA stood out as one of the most abundant polymers in Lenga and Tubul-Raqui. It is widely used in various areas such as the automotive sector, although it is also predominant as a component of fishing nets [30,48], which is highly relevant given the fishing activity in both areas. Polyester and polyurethane were also present, being the most abundant polymers after PA in Lenga, and were likewise present in Tubul-Raqui. One of the main sources of PE is synthetic textiles, with rather varied uses, including outdoor clothing, bags, and backpacks [29,49], while PU, is used mainly for sponges, isolation, coatings, and adhesives, among other uses [50].

PVC was another polymer occurring in both estuaries, known for its high versatility in various applications such as impermeable products, industrial uses like pipes, and food packaging, among others. Despite being one of the most produced types of polymers worldwide, it is considered one of the most dangerous plastics due to the possible release of toxic additives, representing a risk for both the environment and health [51,52,53]. For instance, a study conducted by H. Luo et al. [54] reveals that microplastics can release a diverse range of additives, several of which are categorized as carcinogenic, mutagenic, or endocrine disruptors, posing threats to aquatic life. Furthermore, it has been demonstrated that pristine commercial PVC contains various trace metals used as additives, particularly zinc (Zn) and lead (Pb), the latter of which is known to be highly toxic to most organisms [55], which is important to emphasize, considering that PVC was the most abundant polymer in Tubul-Raqui.

Nevertheless, despite the relevance of these findings, the reported results regarding polymer compositions may be limited due to the low concentrations of MP particles found in both estuaries. As shown in Table 2, other studies have generally reported higher concentrations than those observed in estuaries Lenga (106,9 ± 67.2 particles/kg d.w.) and Tubul-Raqui (49.3 ± 30.9 particles/kg d.w.). For example, Castro-Jiménez et al. [39] reported estuarine sediment concentrations reaching up to two to three orders of magnitude higher (131–13,062 particles/kg d.w.). Although such differences may be attributed to various factors, ranging from methodological aspects to specific contamination events, concentrations remain a key variable that can significantly influence the representativeness and diversity of the polymers identified.

Moreover, beyond the identified MPs, it is also crucial to highlight the particles that are not considered MPs, as they may be intrinsically related to pollution from other sources or polluting agents. This is essential for conducting a more thorough assessment of the scenario and the situation estuaries face, along with the resulting risk. For example, in both estuaries, styrene-grafted cotton stood out. While cotton is considered a natural fiber, it requires various chemical treatments before being used in the manufacture of fabrics and other products [56]. Furthermore, as they contain styrene, which is a chemical product important in the manufacture of several products such as synthetic rubber [57], the fibers cannot be considered of natural origin, but rather synthetic. A similar situation occurs with other compounds present, such as polyamic acid particles, which are related to the formation of polyamides. According to Muñoz-Prieto et al. [58], polyamides are among the most important materials for the engineering polymer industry because their characteristics meet the requirements of multiple industrial applications. Similarly, styrene-divinylbenzene is commonly employed in the synthesis of advanced polymeric materials, encompassing high-performance polymers that range from flexible elastomers to rigid, brittle plastics [59,60]. In both cases, although they are particles that were not classified as MPs, they are still related to the formation of synthetic polymers. As they are present in Lenga and Tubul-Raqui, their possible consequences in the environment must be paid attention to and studied.

Among the potential pathways for MPs pollution, fishing activity stands out as a significant contributor, particularly considering that San Vicente Bay, where the Lenga estuary is located, is a region actively exploited by industrial and artisanal fishermen [61]. This situation is mirrored in the Tubul-Raqui, where artisanal fishing activity is also increasing [23]. The increasing intensity of fishing in these areas suggests that such activity could be a potential source of MPs pollution in both estuaries, along with urban development in the adjacent areas and the direct discharge of partially treated water into the Tubul-Raqui estuary [24]. This could also explain the greater variability of polymers there compared to Lenga, since most MPs originate from terrestrial sources such as domestic waste and wastewater [19]. Therefore, neither estuary can be considered free of pollution, reinforcing the need to research both MPs and non-MP particles to thoroughly understand the complexity of pollution in these ecosystems.

However, it is also important to highlight other transport pathways that are not always considered when assessing pollution sources, which could be contributing to MPs´ pollution levels in both quantitative and qualitative terms. These routes, termed “natural pathways”, involve natural processes known to transport microplastics, including wind, surface runoff, atmospheric circulation, and precipitation [62]. It is important to stress that MPs´ transport can have both short and relatively long ranges, as MPs have been recorded even in remote mountainous basins and glaciers [63]. Additionally, estuary characteristics, such as variations in water levels and flows, as well as the volumes and discharges of the tributary rivers, among other aspects, are key elements that can significantly influence MPs concentrations. For instance, according to Malli et al. [64], during wet seasons, there is an increase in MPs input due to surface runoff, stormwater runoff, and tributary contributions.

Beyond these transport mechanisms, the accumulation of MPs within estuarine sediments is also strongly influenced by sediment characteristics. According to Alava et al. [19], MPs particles tend to accumulate in deposition zones characterized by a high quantity of small, fine-grained particles and high organic matter concentrations, which aligns with the observed correlations in this study. Additionally, a study conducted by Vianello et al. [65] also identified a highly significant correlation between MPs concentrations and sediment particle size, suggesting that finer sediment is associated with higher MPs concentration. While Arias et al. [38] also propose a similar hypothesis, linking it to transport processes, they emphasized the need for caution, as variations in MPs’ inputs could distort the correlation between MPs concentrations and particle size, potentially leading to misinterpretations.

Collectively, these transport and accumulation processes indicate that MPs pollution in the Lenga and Tubul-Raqui estuaries represents a critical environmental concern, given the essential role these ecosystems play in supporting the production and growth of diverse estuarine flora and fauna [66]. Estuarine sediments function as key regulators of carbon, nutrient, metal, and sulfur fluxes; thus, MP contamination may alter sediment properties and biogeochemical cycles [67,68]. Moreover, resident species are at risk, as MPs in aquatic environments have been linked to adverse effects on fish and zooplankton, with potential implications for human health [69]. This concern is amplified by the fact that both estuaries support fisheries for human consumption. Although the MP levels reported here do not provide conclusive evidence of effects, further research is warranted to assess long-term impacts, particularly through trophic transfer. Given the potential environmental and ecological impacts, the implementation of regulatory measures is essential. Despite the significant role that estuaries play, they have received little attention regarding MP pollution in Chile. Recent years, however, have seen progress in national regulations targeting plastic waste, the primary source of MPs in marine environments. Notably, Law no. 21100 (2018), which prohibits the distribution of plastic bags in commercial establishments nationwide, has been fully enforced since 2019. Law no. 21368 (2021) further regulates single-use plastics and disposable plastic bottles, mandating restrictions on items such as straws, cutlery, and expanded polystyrene, while promoting reusable alternatives and recycled content in beverage bottles. Additionally, Law no. 20920 (2016) introduced the extended producer responsibility (EPR) framework, requiring producers of priority products to finance and organize waste management systems, promoting recycling and circular economy practices.

While these legislative measures represent notable progress, it is essential to evaluate the implementation of additional regulations and explore new strategies to reduce the contribution and release of plastics into estuarine and other ecosystems. Given that MPs are largely the result of microplastic fragmentation and considering that their removal from the environment is a complex and virtually impossible task, it is crucial to focus efforts on reducing the use of non-essential plastics, such as single-use items and packaging, as well as ensuring the proper management of those plastics that are more difficult to replace. Complementary to these actions, it is also essential to investigate preventive measures aimed at reducing the continuous release of MPs into natural environments.

5. Conclusions

The two estuaries analyzed in this study exhibited microplastic contamination, with a markedly higher total particle abundance observed in the Lenga estuary. This disparity in particle abundance may be largely attributed to the increased anthropogenic pressure faced by the Lenga estuary in comparison to the Tubul-Raqui estuary. Nevertheless, both ecosystems face similar anthropogenic activities in their vicinity, such as fishing and urbanization, which may serve as significant sources of MPs input. In both cases, the prevalent forms of microplastics identified were fibers and foam. In Lenga, the most abundant polymer types were polyamide, polyester, and polyurethane, which were also present in Tubul-Raqui; however, polyvinyl chloride was the most abundant there. Given the absence of baseline data on MPs pollution in these Chilean estuaries, a situation that replicates around the world, this study underscores the urgent need for long-term monitoring programs and ecological risk assessments, particularly regarding potential impacts on aquatic biodiversity. Furthermore, it is imperative to develop and implement integrated management strategies and preventive measures to mitigate the primary sources of MPs pollution, in line with global efforts to address marine plastic contamination.